Leyline Binding's domain ability doesn't change its mana value, which is always 6.

If Leyline Binding leaves the battlefield before its enters-the-battlefield ability resolves, the target permanent won't be exiled.

Auras attached to the exiled permanent will be put into their owners' graveyards. Equipment attached to an exiled creature will become unattached and remain on the battlefield. Any counters on the exiled permanent will cease to exist.

If a token is exiled, it ceases to exist. It won't be returned to the battlefield.

Leyline Binding's last ability is a single ability that creates two one-shot effects: one that exiles the permanent when the ability resolves, and another that returns the exiled card to the battlefield immediately after Leyline Binding leaves the battlefield.

Domain abilities count the number of basic land types among lands you control, not how many lands you control or how many of any type.

The basic land types are Plains, Island, Swamp, Mountain, and Forest. Land types other than basic land types (such as Desert) don't contribute to domain abilities.

In a multiplayer game, if Leyline Binding's owner leaves the game, the exiled card will return to the battlefield. Because the one-shot effect that returns the card isn't an ability that goes on the stack, it won't cease to exist along with the leaving player's spells and abilities on the stack.